Bipolar membranes (BPMs) are gaining interest in energy conversion technologies. These membranes are composed of cation- and anion-exchange layers, with an interfacial layer in between. This gives the freedom to operate in different conditions (pH, concentration, composition) at both sides. Such membranes are used in two operational modes, forward and reverse bias. BPMs have been implemented in various electrochemical applications, like water and CO2 electrolyzers, fuel cells, and flow batteries, while BPMs are historically designed for acid/base production. Therefore, current commercial BPMs are not optimized, as the conditions change per application. Although the ideal BPM has highly conductive layers, high water dissociation kinetics, long lifetime, and low ion crossover, each application has its own priorities to be competitive in its field. We describe the challenges and requirements for future BPMs, and identify existing developments that can be leveraged to develop BPMs toward the scale of practical applications.

Bipolar membranes (BPMs) are recently emerging as a promising material for application in advanced electrochemical energy systems such as (photo)electrochemical CO2 reduction and water splitting. BPMs exhibit a unique property to accelerate water dissociation and ionic separation that allows for maintaining a steady-state pH gradient in electrochemical devices without a significant loss in process efficiency, thereby allowing a broader catalyst material selection for the respective oxidation and reduction reactions. However, the formation of high-performance BPMs with significantly reduced overpotentials for driving water dissociation and ionic separation at conditions and rates that are relevant to energy technologies is a key challenge. Herein, we perform a detailed assessment of the requirements in base materials and optimal design routes for the BPM layer and interface formation. In particular, the interface in the BPM presents a critical component with its structure and morphology influencing the kinetics of water dissociation reaction governed by both electric field and catalyst driven mechanisms. For this purpose, we present, among others, the advantages and drawbacks in the utilization of a bulk heterojunction formed in 3D structures that recently have been reported to demonstrate a possibility of designing stable and high-performance BPMs. Also, the outer layers of a BPM play a crucial role in kinetics and mass transport, particularly related to water and ion transport at electrolyte-membrane and membrane-catalyst interfaces. This work aims at identifying the gaps in the structure-property of the current monopolar materials to provide prospective facile design routes for BPMs with excellent water dissociation and ionic separation efficiency. It extends to a discussion about material selection and design strategies of advanced BPMs for application in emerging electrochemical energy systems.